He was born in Lancaster, Ohio. He was the older brother of John Sherman, the Senator who sponsored the Sherman Antitrust Act. Sherman's father, Judge Charles Sherman, died when Tecumseh was nine years old. The boy was informally "adopted" by a Lancaster neighbor, attorney Thomas Ewing[?], who served as a U.S. Senator from Ohio and as U.S. Secretary of the Interior. Ewing secured Sherman's apppointment to the United States Military Academy at West Point, from which he graduated sixth in his class of 1840. He entered the army as a second lieutenant and served through the end of the Mexican American War, during which he was stationed in California.

He resigned his military commission and became president of a bank in San Francisco. The bank failed in a financial panic in 1857. Sherman accept a job offered to him by two of his Southern army friends, P.G.T. Beauregard and Braxton Bragg, as the first president of Louisiana Military Seminary. He served in that post until the start of the American Civil War, when he resigned and went back north. It is an especially delicious irony that Louisiana Military Seminary later became Louisiana State University[?]. Thus the Yankee general Southerners most love to hate was the first president of what is now one of the most prestigious and beloved Southern universities. Beauregard and Bragg, of course, became Confederate generals during the Civil War.
Sherman accepted a commission as a colonel in the U.S. Army and was one of very few Union officers to actually distinguish himself at the first Battle of Bull Run.

He was made a brigadier general and put in command of a military department headquartered in Louisville, Kentucky. During his time in Louisville Sherman went through a personal crisis that has been variously described as a "nervous breakdown" or "insanity." Without question he was working too hard, drinking and smoking too much and suffered some kind of collapse which made it necessary for him to go home to Ohio to recuperate. Yet just six months later he was a brilliant and brave major general serving under U. S. Grant at the April 1862 Battle of Shiloh. He suffered two slight wounds during that two-day battle in west Tennessee and had four horses killed from under him.

Sherman developed close personal ties to Grant during the approximately two years they served together. At one point not long after the Battle of Shiloh, Sherman persuaded Grant, who was being badly treated by his commander, General H. W. Halleck[?], not to resign from the army. The careers of both officers ascended considerably after that time. They shared in the glory of conquering Vicksburg in July 1863 and at the Battle of Lookout Mountain[?] in Chattanooga[?]. In much later years Sherman said simply, "I took care of Grant when he was drunk, and he took care of me when I was crazy."

When Lincoln called Grant east in the spring of 1864 to take command of all Union armies, Grant appointed Sherman his successor as commander of the western theatre of the war. His siege and capture of Atlanta, Georgia and subsequent March to the Sea from Atlanta to Savannah in the autumn of 1864 sealed Sherman's position as one of the great Union heroes of the Civil War. When Grant became president in 1869, Sherman became the top general in the U.S. Army and served in that post until his retirement.

In 1875 Sherman published his two-volume Memoirs, a minor classic, marked by a forceful, lucid style, and the strong opinions for which Sherman became famous.

Sherman retired from the army in 1884, and lived most of the rest of his life in New York City. He was devoted to the theatre and much in demand as a colorful speaker at dinners and banquets in New York and elsewhere.